In optics, spatial periodic modulation of refractive index of an optical material provides general means for spectral control of the transmission, reflection or diffraction of light. As an example, fiber Bragg gratings (“FBG”) are widely employed in the sensing and telecom applications as narrow-band spectral filters. FBGs are generally made by forming a periodic refractive index change along the fiber length by applying periodic laser exposure in the photosensitive core waveguiding region. Bragg gratings function as fundamental filter and/or sensor components used in many conventional optical circuits, both in fiber and planar lightwave circuits. In another manifestation of fiber-based gratings, long period gratings (“LPG”) can be formed by similar means, with the purpose of coupling light from/to the fiber core to/from the cladding modes.
Several methods have been applied to fabricate Bragg and long-period gratings within existing waveguide structures, including various types of optical fibers and planar structures.
For example, Hill et al. discloses a FBG structure in U.S. Pat. No. 4,474,427. A laser fabrication method for a laser propagating inside the core of an optical fiber is taught, and requires partial reflection to create a counter-propagating beam that upon interference with the incoming beam, forming a relatively narrow spectrum grating with Bragg reflection only at the wavelength of the writing laser.
U.S. Pat. No. 4,807,950 to Glenn et al. discloses FBG fabrication by two-beam laser interference (holography) with an ultraviolet laser source. However, the method requires a pre-existing waveguide (i.e a photosensitive core) in which the external laser can interact and modify the refractive index change.
U.S. Pat. No. 5,104,209 to Hill et al. describes fiber grating fabrication by an amplitude mask. Similar with the previous methods, an existing waveguide is required to modify the refractive index of the core and thereby form a grating (in the photosensitive core). As well, a point-by-point method is a relatively slow fabrication method, is generally directed to coarse (long period) structures, and requires an existing waveguide and a mask.
A further improvement on the above point-by-point method is disclosed by Snitzer et al. in Canadian Patent No. 2,372,939 (see PCT Patent No. WO9409369; also European Patent No. 1,197,771 and U.S. Pat. No. 5,351,321), where an amplitude mask technique comprising a series of squares apertures is used. This technique also requires an existing waveguide and uses an ultraviolet light source.
Hill et al. in U.S. Pat. No. 5,367,588 teaches FBG fabrication by a phase mask. The method improves the optical stability over the holographic interference technique, by employing a microstructured diffractive phase mask to create two interfering laser beams from one beam, but only in the proximity of the phase mask device. However, this method is inflexible in comparison with the holographic method, for example, when multiple wavelength Bragg grating devices are required. Separate generally high-cost phase masks are required for each Bragg wavelength, and relatively time-consuming multiple laser exposures with various the phase masks are then necessary to produce the desired multi-wavelength spectral response. This phase mask technique also requires an existing waveguide and uses an ultraviolet light source.
Further, Albert et al. in U.S. Pat. No. 6,256,435 teaches a method of forming Bragg gratings in a planar lightwave circuit (“PLC”). The technique has a disadvantage over the formation of FBGs in that Bragg gratings formed in planar lightwave circuits have weaker reflection due to lower photosensitivity of glass materials contained in the planar light circuit.
These various techniques of Bragg grating fabrication in optical materials can be generally classified as one-dimensional (1D) in the case of FBG and two-dimensional (2D) devices in the case of PLCs.
Laser direct writing, for example with femtosecond duration laser pulses, define a new methodology for generating various types of photonic devices internally in bulk transparent material, with laser interactions confined in or near the laser focal volume. In this way, three-dimensional (3D) photonic devices may be fabricated. Various types of lasers are used to alter the refractive index of a material in bulk materials, for example, to create buried waveguides, in a manner that is well known.
For example, see Mourou et al. in U.S. Pat. No. 5,656,186, which describes ultrashort laser interactions with materials. No internal waveguide writing in bulk material was described.
Davis et al. in “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21, 1729-1731 (1996) discloses a method of forming buried optical waveguides with ultrashort duration lasers.
Wei et al. (see M. Wei, K. P. Chen, D. Coric, P. R. Herman, J. Li, F2-laser microfabrication of buried structures in transparent glasses, Photon Processing in Microelectronics and Photonics, SPIE Proc. 4637, Photonics West, 20-25 Jan. 2002, p. 251-257) presents an alternative means of forming buried optical waveguides in transparent glasses that also employs scanning of a focused laser beam, but with much longer laser pulse duration of approximately 15 nanoseconds.
Borrelli et al. in U.S. Pat. No. 6,977,137 (2005) discloses waveguide writing in three dimensions and various devices.
Such ultrashort laser writing of optical circuits is promising as a fabrication method for creating compact optical circuits by forming devices in multi-layers or other geometries exploiting the full 3D physical space in comparison to planar light circuits (2D) or fiber optics (1D). Various devices such as power splitters, directional couplers, and multi-mode interference (MMI) power splitters are possible. However, a roadblock in the development of such 3D laser writing processes has been the inability to generate basic grating filter/reflectors devices within the 3D waveguide structures formed by the laser writing method.
Mihailov et al. in U.S. Pat. No. 6,993,221 teaches the combination of ultrafast laser and phase mask exposure to generate short-pulse laser interference inside the waveguide core of an optical fiber and thereby form a permanent refractive index change with characteristic period greater than half of that of the mask. However, this technique has only been demonstrated to be successful in a pre-existing waveguide (optical fiber).
Kalachev et al. in Journal of Lightwave Technology 23, 8, 2568-2578 (2005) discloses a femtosecond ultraviolet light source method for fabricating a long period fiber grating in pre-existing waveguide. However, this point-by-point method only provides low spatial modulation and therefore is very limited in the type of gratings that can be formed. This technique also requires an existing waveguide for laser formation of a grating.
Martinez et al. in “Direct writing of fiber Bragg gratings by femtosecond laser”, Electron. Lett. 40, 19 (2004), describes point-by-point writing of FBG with a femtosecond laser (150 fs, 1 kHz). The method employs a scan technique, but requires also an existing waveguide in the fibers.
Laser waveguide writing in crystalline materials was demonstrated by Nolte et al. in “Waveguides produced by ultrashort laser pulses inside glasses and crystals”, Proc. of SPIE Vol 4637, 188-196 (2002), and “Femtosecond writing of high quality waveguide inside phosphate glasses and crystalline media using a bifocal approach”, Proc. of SPIE, vol. 5340, 164-171 (2004), and discloses waveguide formation in crystalline materials. See also PCT No. WO 2005/040874 to Khruschev et al.
There are also examples of longer pulse duration lasers (<1 ms) being successfully applied to 3D fabrication in optical materials, for example, of volume gratings (for example, see J. Zhang, P. R. Herman, C. Lauer, K. P. Chen, M. Wei, 157-nm laser-induced modification of fused-silica glasses, in Laser Appl. in Microelectronic and Optoelectronic Manuf. V, SPIE Proc. 4274, Photonics West, 20-26 Jan. 2001, pp. 125-132) or buried optical waveguides (for example, see M. Wei, K. P. Chen, D. Coric, P. R. Herman, J. Li, F2-laser microfabrication of buried structures in transparent glasses, Photon Processing in Microelectronics and Photonics, SPIE Proc. 4637, Photonics West, 20-25 Jan. 2002, p. 251-257.), although combination of gratings and waveguides were not demonstrated.
Yamaguchi discloses in Japanese Patent Application No. (2000)-144280 a laser method to generate an optical waveguide in doped glass with first-order Bragg gratings responses. The Bragg responses are induced during laser scanning by periodically changing the intensity of the laser light, the diameter of laser light at the focusing point, or relative moving speed to generate relatively smooth waveguides with periodic modification of refractive index.
In summary, there has been considerable development in the fabrication of optical/photonic circuits by pulsed lasers. The basic grating fabrication techniques, including (i) the use of amplitude/phase masks, (ii) holographic interference of two beams, and (iii) point-by-point grating writing, suffer from numerous disadvantages, including being expensive, time-consuming, and not readily applicable to 3D application. Furthermore, most known optical devices have been based on smoothly connected optical waveguides, absent of periodic structures, or where grating structures have been desired the fabrication techniques have relied upon existing waveguide structures before fabricating gratings. On the basis of the foregoing, what are needed are optical devices comprising gratings structures and waveguides and an improved means of fabricating same.